Ethanol is both a desirable chemical product and feedstock. Ethanol is steadily becoming a promising alternative to gasoline throughout much of the world. Ethanol is also useful for producing ethylene, which is a leading petrochemical in terms of production volume, sales value, and number of derivatives. Ethanol can also be converted to butadiene, a precursor to synthetic rubbers, by the Lebedev process. An economically viable process that can produce ethanol and/or ethylene from methane (or other advantaged carbon-containing feedstock) would therefore be highly desirable. However, existing process schemes that can accomplish this transformation suffer from several drawbacks.
For example, it is known to convert methane directly to ethylene via oxidative coupling, but this route often suffers from low yields (high trade-off between conversion and selectivity), frequently requires expensive oxygen generation facilities, and produces large quantities of undesirable carbon oxides. In addition, non-oxidative methane conversion is equilibrium-limited, and temperatures of 800° C. or more are needed for methane conversions greater than a few percent.
A potentially more attractive route involves converting methane or other carbon-containing feedstock to a mixture comprising carbon monoxide and hydrogen (the mixture being conventionally referred to as “syngas”), converting the syngas to a mixture of oxygenates, and then converting the oxygenates to olefins. See, e.g., US 2005/0107481 A1, US 2008/0033218 A1, and US 2007/0259972 A1, which disclose aspects of converting syngas to a mixture comprising C1 alcohol and C2 alcohol, and then converting the mixture to a product mixture comprising ethylene and propylene. According to those references, approximately 100% of the mixture's ethanol can be selectively converted to ethylene. The mixture's methanol, in contrast, produces (i) ethylene and propylene, in approximately equal amounts, and (ii) a significant amount of by-products. The by-products can include, e.g., one or more of hydrogen, water, alcohols, carboxylic acids, ethers, carbon oxides, ammonia and other nitrogenated compounds, arsines, phosphines, and chlorides. The by-products can also include hydrocarbons, such as one or more of C4 to C30 olefins, acetylene, methyl acetylene, propadiene, butadiene, butyne, and the like, and combinations thereof.
Other syngas-based schemes have been proposed which can produce ethylene in higher selectivity, see, for example, San et al. Energy & Fuels 2009, 23, 2843-2844. However, these require the addition of methanol or dimethyl ether co-feeds to satisfy process stoichiometry.
Other references of interest include: Mixed Alcohol Synthesis Catalyst Screening, M. Gerber et al. Pacific Northwest Laboratory 2007, PNNL-16763; US 2015-0158785; Cheung et al. Angew. Chem. Int. Ed. 2006, 45, 1617-1620; Yang et al. Catalysis Today 2011, 164, 425-428; San et al. Energy & Fuels 2009, 23, 2843-2844; WO 2009/077719; Li et al. ChemSusChem 2010, 3, 1192-1199; Bhan et al., J. Am. Chem. Soc. 2007, 129, 4919-4924; and US 2008/016833.
There is, therefore, a need for an efficient process for the conversion of syngas to ethanol (and, if desired, eventually ethylene via ethanol dehydration), which minimizes methanol (or other oxygenates) and/or propylene byproduct production, and also only requires a syngas feed.